In recent years, oil and gas has been recovered from subsea wells in very deep water, of the order of over 1 km. This poses many technical problems in drilling, securing, extracting and abandoning wells in such depths.
In the event of a failure in the integrity of the well, wellhead apparatus control systems are known to shut the well off to prevent dangerous blow-out, or significant hydrocarbon loss from the well. Blow-out-preventers (BOPs) are situated at the top of subsea wells, at the seabed, and can be activated from a control room to shut the well, or may be adapted to detect a blow-out and shut automatically. Should this fail, a remotely operated vehicle (ROV) can directly activate the BOP at the seabed to shut the well.
In a completed well, rather than a BOP, a “Christmas” tree is provided at the top of the well and a subsurface safety valve (SSV) is normally added, “downhole” in the well. The SSV is noimally activated to close and shut the well if it loses communication with the controlling platform, rig or vessel.
Despite these known safety controls, accidents still occur and a recent example is the disastrous blow-out from such a subsea well in the Gulf of Mexico, causing a massive explosion resulting in loss of life, loss of the rig and a significant and sustained escape of oil into the Gulf of Mexico, threatening wildlife and marine industries.
Whilst the specific causes of the disaster are, at present, unclear, some aspects can be observed: an Emergency Dis-connect System (EDS) controlled from the rig failed to seal and disconnect the vessel from the well; a dead-man/AMF system at the seabed failed to seal the well; subsequent Remotely Operated Vehicle (ROV) intervention also failed to activate the safety mechanisms on the BOP. Clearly the conventional systems focused primarily on the blow-out-preventer did not activate at the time of the blow-out and also failed to stem the tide of oil into the sea after control communication was lost with the rig.